Harlan Lachman wrote:

Actually Peggy, up here in Vermont, they now build in vents for the
boats, usually one at each end or one on the stern and one on either
side by the mast.

Years ago, as one of the people in charge of VT's energy efficiency
programs, I had to learn a lot about ventilation. My guess is most of
the vents in most of the boat covers don't work real well.

Effective ventilation requires low intakes and high exhausts and a lot
more free vent area than anyone is going to put on a boat. Some say
canvas is porous enough to allow moisture to migrate.

My guess it is best to keep boats in climates where covers are
unnecessary.

harlan

I'll have to use my heavy canvas tarp again this winter for my
trailerable mini-cruiser sailboat, but next spring, I'll add an
additional couple of feet to my garage so the boat can be snug and dry
for the five months of Nov-March.